1. Field of the Invention
The present invention relates to an internal absolute laser emission wavelength calibration method and apparatus, and particularly to a laser having a discharge chamber wherein a molecular species is introduced, e.g., carbon- or oxygen- containing molecules, which reacts to form photoabsorbing species, such as atomic carbon or molecular oxygen, having one or more photoabsorption lines within the output emission spectrum of the laser, e.g., an ArF-excimer laser emitting around 193 nm, for calibrating an absolute wavelength of the laser output emission.
2. Discussion of the Related Art
The formation of structures in the range of 100 nm by employing optical microlithography requires deep ultraviolet (DUV) light sources as exposure tools. Today, one of the most efficient light sources in this spectral range is the Argon Fluoride (ArF)-excimer laser emitting light around a wavelength of 193.3 nm and having a bandwidth (FWHM) of approximately 430 pm. KrF-excimer lasers emitting around 248 nm are also widely used. F2-excimer lasers emitting around 157 nm are coming into use and are believed to become more widely used as the formation of smaller structures on silicon substrates is desired.
Resolvable structures are generally formable if their sizes are approximately on the order of the emission wavelength of the photolithographic radiation source being used, or larger. Special techniques have thus been developed to enable the formation of structures half the size of the source wavelength. A first technique uses refractive imaging optics to deliver the light from the excimer laser to the substrate for processing. In this technique, exposure of wafer substrates is carried out by using a complex system of lenses. This high resolution imaging process uses a drastically narrowed emission bandwidth for exposure to reduce dispersion effects otherwise associated with the refractive elements. Typical bandwidths (FWHM) used are less than 0.6 pm.
Additionally, the imaging optics used in this first technique are particularly selected for use with the output emission wavelengths of the radiation source. The optical system employs precise wavelength calibration having an accuracy within around xc2x10.1 pm together with an absolute wavelength stability within xc2x10.1 pm.
A second technique for reducing the resolvable size of structures formed on a silicon substrate employs reflective optical elements for the imaging process. This catadioptric technique has greater bandwidth flexibility, and typical bandwidths used are in the range of 25-50 pm.
To better enable precise wavelength selection for lithography, the emission wavelength of the exposure source, e.g., an excimer laser, is calibrated regularly. This calibration can be performed by tuning the narrowed excimer laser emission wavelength through a known absorption or inter-level transition line of, e.g., an atomic or molecular material filling a sealed module and having one or more resolvable absorption lines around the emission wavelength of the laser. An absorption or inter-level transition is detected, e.g., by a defined change in detected signal intensity through the material or an opto-galvanically modified potential difference between two points within the module. The laser is then tuned to a known absolute wavelength because the absolute wavelength of the absorption or inter-level transition line is known and any tuned offset of the laser output emission wavelength therefrom is substantially determinable.
Each of these techniques undesirably uses an additional sealed gas-filled module to perform absolute wavelength calibration. The additional module increases the size of a laser arrangement employing the calibration technique. Alignment of the module and supply and maintenance of the module and its peripheral monitoring equipment unsatisfactorily consume space, time and cost.
It is desired to have a method and apparatus for accurately and efficiently calibrating an absolute wavelength of an excimer laser. A laser system is thus provided comprising a discharge chamber filled with a gas mixture including a laser gas and trace amounts of a photoabsorbing species. The photoabsorbing species has at least one photoabsorption line within an output emission spectrum of the laser. The laser system further includes a wavelength selection unit and a detector. The output emission of the laser is narrowed and tuned within the broadband emission spectrum of the laser including the at least one photoabsorption line. An absolute wavelength of the narrowed emission can be calibrated when the narrowed emission is tuned through the photoabsorption line.
The photoabsorbing species is preferably either atomic carbon or molecular oxygen. The atomic carbon or molecular oxygen is preferably provided by first introducing trace amounts of carbon-containing molecules or oxygen-containing molecules, respectively, together in the discharge chamber with the laser gas. The carbon- or oxygen-containing molecules then react within the discharge chamber and atomic carbon or molecular oxygen are produced. A method of performing the calibration is provided along with means for performing the calibration steps.
Preferably, a processor communicates with the detector and the wavelength selection unit to automatically perform the calibration. Also preferably, one of tetrafluorocarbon, trifluoromethane, difluoromethane, fluoromethane, methane, hydrocarbon and partially fluoronated hydrocarbon, ethane and partially fluoronated ethane, and propane and partially fluoronated propane molecules are selected as the carbon-containing molecules or one of carbon dioxide, carbon monoxide or oxygen are selected to provide the oxygen within the discharge chamber. When carbon is selected, a photoabsorption line of atomic carbon at 193.0905 nm is compared with the spectral position of the narrowed emission of an ArF-excimer laser to determine its absolute wavelength. When oxygen is selected, at least one photoabsorption line of molecular oxygen (the main absorption lines are 193.114 nm, 193.292 nm and 193.493 nm) is compared with the spectral position of the narrowed emission to determine the absolute wavelength. The absolute wavelength of the ArF-excimer laser may be calibrated with an accuracy within around 0.1 pm, and may be generally calibrated within an accuracy of around ten percent of the narrowed bandwidth of the laser system being used.